The detection of explosives has become a highly important issue in recent years, especially because of the increasing occurrence of terrorist activities all over the world. Searching for explosives by security personnel is nowadays a matter of course in many public places and installations such as airports, government edifices, army and police installations and the like. In many instances such searches are carried out by manual inspection of people's belongings and may occasionally also involve the employment of dogs trained to sniff explosives. It is also customary to X-ray personal belongings in order to detect suspicious objects; but generally, indications obtained in this way are considered inconclusive and as a rule, when something suspicious is detected a follow-up manual inspection is required.
Until now no reliable method for swift and sensitive detection of explosives has been developed.
Nuclear resonance fluorescence is a process in which a nucleus is raised to an excited state by absorbing an incident photon. The excited nucleus may decay by emission of either a photon - (.gamma.,.gamma.) reaction known as nuclear resonance scattering (NRS) or by a particle emission, e.g. a (.gamma.,p) reaction. Such decay or de-excitation is characterized by the emission of .gamma.-radiation of specific energies which correspond to certain discrete energy levels of the atomic nucleus and which are atomspecific. Consequently in NRS it is possible to detect a given substance in a mixture by irradiating the mixture with .gamma.-rays having an energy that corresponds to the energy level to which the sought-after atom could be excitable. If that atom is present in the mixture there occurs a resonant absorption which in turn produces fluorescence detectable by photon detectors that are in common use.
The main use of NRS is in research, e.g. for determining half-lives of excited nuclear state of various atoms, but some analytical methods based on NRS were developed for application in medicine, e.g. for detection of excessive amounts of iron in the liver, and also in the mineral industry, e.g. for detection of copper and nickel in minerals. These methods are, however, inapplicable in detecting nitrogenous explosives since the partial electromagnetic widths for ground state transitions from most excited states of nitrogen is very low and, therefore, NRS thereof is barely detectable.